Merchants in Oleshie?, local factors for Isidor, who is based in Novgorod.
The party, as part of its search for the hoard of Zmei Gromovoi, headed to Crimea? to initiate contact with the Mongol Khan. Before the party's return to Oleshie?, their rooms were torched, apparently by magic, and they are presumed dead. The timing is appropriate for this to have been done by Ambassador Said.
Perhaps the most critical thing, assumed to have led to their deaths, is that in the message to Nogai Khan, they concealed the statement "we know how to decipher the demon's writing".